GEOLOGY | FOSSILS | key fossil groups
plants & FUNGI
The Fungi are not plants, they are of a separate kingdom that is more closely related to multi-celled animals (Metazoa) than to multi-celled plants (Metaphyta). We include them in this section in a slavish nod to Botanical tradition. The earliest fungi fossils are found in the Devonian and Carboniferous but phylogenetic evidence suggests that fungi existed in the Precambrian as microscopic parasites.
The first land plants were the ancestors of the mosses, liverworts and hornworts (bryophytes). Fossils of these plants, although rare, can be found in the rock record. The earliest bryophytes lived on wet mudflats during the Devonian, but it is likely that they colonised the land during the late Silurian.
Vascular plants made their debut in the tree of life in the late Silurian. They resembled the green algae from which they evolved. Cooksonia, the first to evolve, was a small, 60 mm high plant that lacked leaves, but had spherical spore sacs at the ends of its branched stems. A little later, Zosterophllophytina, the ancestor of the clubmosses, and Rhyniophytina, ancestor to leaf-bearing Trimerophytina, appeared.
Fungi are classified into various phyla according to the patterns of reproductive structures present. The first good examples of fossils with reproductive structures intact were from the Devonian and Carboniferous periods. The Ryhnie Chert Lagerstätte from Aberdeenshire in Scotland shows evidence of hyphae mats and the first example of fungi-plant symbiosis. Present in the Rhine Chert are: chytridiomycetes, ascomycetes, oomycota and glomeromycetes.
Devonian and Carboniferous fungi appeared to have acted as decompress of plant detritus and parasitic invaders of living, healthy plant tissues.
Although the first fungi fossils with intact reproductive structures date from the Devonian and Carboniferous, the presence of lichens in 600 Ma deposits in China, suggest that fungi were present in the Precambrian. Lichens area symbiotic consortium of two organisms: Cyanobacteria (photosynthesis) and Fungi (nutrients, protection and moisture).
Image: silicified stem of Aglaophyton from the Devonian Rhynie Chert.
© UCMP Berkeley, Ca, USA.
Annularia (form genera: leaf of Calamites)
The study of fossil plants falls into two academic disciplines:
Palaeobotany concentrates on the macroscopic remains of plants eg, stems, leaves, roots, flowers, wood, fruits and seeds, whereas Palynology is predominantly the study of microscopic plant remains eg. pollen and spores. This page of the OGG website concentrates on the macroscopic. There is a page on microfossils where palynology is explored in more detail.
Image: Rhynia gwynne-vaughanii, (Aberdeenshire) © NHM.
Invading the LAND
The first plants on land appear to have been the bryophytes. Bryophytes include: hornworts, liverworts and mosses. The paucity of bryophytes in the fossil record is attributed to low preservation potential. However, it should be noted that it is difficult to distinguish early plant fossils.
A waterproof cuticle over their leaves and stems adapt bryophytes to terrestrial life.
A few hornworts and mosses possess stomata, an adaptation to reduce water loss. Stomata are absent in liverworts.
Some larger mosses and liverworts have a very simple vascular conducting system.
Various bryophytes are able to desiccate completely during arid conditions, only to rehydrate when rain falls.
Image: Bryophte fossil from Apple Bay Flora, Vancouver. Credit: A. Tomescu
the first forests
The Lycophytes represent a wide range of extinct and living plants that have contributed important data on evolutionary trends in primitive vascular plants. The earliest lycophytes included Baragwanathia and Protolepidodendron, dating from the early Devonian Period. Both were small herbaceous plants. During the Carboniferous Period, which followed (beginning 358.9 million years ago), the treelike forms of the Lepidodendrales appeared.
Over the years, fossil parts of lepidodendronic plants have been discovered and assigned by taxonomists to so-called form genera, or organ genera: Lepidophyllum is the form genera term for detached leaf fossils, Lepidostrobus for fossil cones (pl strobili). These form genera are now recognized as portions of one main fossil genus designated Lepidodendron. See Lepidodendron form genera plate below).
Some other lycophytes co-existing with the tree lycophytes were small herbaceous plants that resembled modern Lycopodium and Selaginella species.
Image: Lepidodendron (Carbonfiferous).
Lepidendrons (scale trees) are the extinct ancestors of quillworts (Isoetes)
The lycophyte fossil, Calamites genus, includes the form genera Annularia (leaves) and Calamities (stem)
Lepidendrons reached greatest diversity in the forest swamps of the Late Carboniferous
The forests of the Carboniferous swamps could have been easily flattened by high winds and storms due to the weak structure of lycophyte trunks
Lepidendrons had only a thin central core of wood surrounded by a thick layer of tissue
Calamites are the 20 metre high ancestors of modern horse tails
The ferns and their relatives first appear in the fossil record some 360 million years ago in the late Devonian period. They diversified into many of the modern fern families and species during the “great fern radiation” of the Cretaceous period, from 145 to 66 MYA. Like the clubmosses, the ferns are early vascular plants that rely on water for sexual reproduction because they have free-swimming sperm. Because of this, they are limited during reproduction to relatively moist environments.
Image: Alethopteris serlii St Clare, PA, USA.
The gymnosperms originated from a common ancestor from the late Devonian, early Carboniferous. by the Permian all of the features of gymnosperms were highly developed.The gymnosperms are divided into six phyla. Organisms that belong to the Cycadophyta, Ginkgophyta, Gnetophyta, and Pinophyta phyla are extant while those in the Pteridospermales and Cordaitales phyla are now extinct. It is a diverse sub-kingdom, containing cycads, and ginkgos. By far the most abundant phylum is the conifers: pines, spruces, firs, hemlocks, cypresses, cedars, junipers and redwoods.
Gymnosperms possess several key evolutionary innovations compared to earlier groups such as the lycophytes and ferns. Gymnosperms are woody plants, either shrubs, trees, or, rarely, vines (some gnetophytes). They differ from flowering plants in that the seeds are not enclosed in an ovary but are exposed within any of a variety of structures, the most familiar being cones.They produce sperm-containing pollen, this innovation has negated water for sexual reproduction. Seeds that can be dispersed by vectors such as wind, water, or by other organisms, allowing rapid colonisation.
Image: Cone of Pinus ebgekgardtii. Carboniferous.
A SPLASH OF COLOUR
Flowering plants (angiosperms) are the most successful plants to have evolved. Of the 300 k species of plants in the world today, about 250 k are angiosperms. The angiosperms were thought to have evolved in tropical regions during the Cretaceous and, by the Palaeogene, they had spread globally, however recent evidence points to an evolutionary appearance at the beginning of the Mesozoic.
The oldest known fossil angiosperms were found in Early Cretaceous rocks in America and Russia. They are known from their leaves, pollen, seeds, branches, but not their flowering parts. Flowers are short-lived, delicate and rarely fossilised. The oldest known petals come from middle Cretaceous rocks in North America and are related to modern day Magnolia.
Image: Fossilised buckthorn flowers found in shales of the Salamanca Formation (Cretaceous-Paleogene) Chubut Province, Patagonia, Argentina
credit: Nathan Jud/Cornell University (USA)